1. Field of the Invention
The present invention relates to a switch that mainly operates on a millimeter waveband.
2. Description of the Related Art
FIGS. 29 and 30 are diagrams showing the general circuit structures of the conventional millimeter waveband switch. In the figures, T denotes a field effect transistor (FET) which is used as a switching element, P1 and P2 are input and output terminals, L1 is a transmission line, V1 is a control voltage supply terminal, and D is a diode.
The switch that operates on the millimeter waveband is generally structured in such a manner that the FET or the diode is arranged in parallel to the transmission line (corresponding to L1 in the figures) through which a signal passes for the purpose of reducing loss when the switch is on.
In the conventional structures, for example, in the case of the structure shown in FIG. 29, the isolation characteristic when the switch is off depends on the on-resistance (Ron) value of the switching element which is arranged in parallel to the transmission line. FIGS. 31 and 32 are diagrams showing the circuit structures of the conventional millimeter waveband switch that aims at high isolation. As shown in FIGS. 31 and 32, it is required that two or more switching elements are arranged in parallel in order to aim at the high isolation.
Also, as the structure of the switch for obtaining the high isolation when the switch is off, there is a structure in which inductance that resonates with an off capacitance when the switch is off at a desired frequency is arranged in series (for example, refer to JP 11-284203 A). FIG. 33 is a diagram showing the circuit structure of the conventional millimeter waveband switch in which the inductance is arranged in series with the switch in order to aim at the high isolation.
However, the conventional art suffers from the following problem.
In the conventional switch having the above circuit structure, the isolation characteristic is improved. However, there arises such a problem that the passing loss increases due to the on-resistance of the switching element when the switch is on.